Work carriers of the type concerned are used in an automobile assembly line for conveyance of a plate-like work, such as vehicle door, to an assembling station where a window glass and other parts are assembled with the door, or to a desired station in another processing line. Typical examples of such work carriers are disclosed in Japanese Utility Model Registration No. 2571180 (corresponding to JP 6001158U) and Japanese Patent Laid-Open Publication No. HEI 11-348777 (JP 1134877 A).
The work carrier disclosed in JP 6001158U includes a fixed clamp member disposed on a base and a movable clamp member disposed via a bracket on the base so that a vehicle door can be clamped at opposite vertical edges thereof by and between the fixed and movable clamp members. The movable clamp member is integrally connected to a slide rod slidably mounted via a compression coil spring to the upper end of a support lever, the lower end of the support lever being pivotally connected to the bracket. The support lever is driven by a link mechanism to undergo pivotal movement toward and away from the fixed clamp member. When the support lever is driven to move toward the fixed clamp member, the movable clamp member is forced into pressure contact with one vertical edge of the door to the extent that an opposite vertical edge of the door is forced against the fixed clamp member. Thus, the door is clamped between the fixed and movable clamp members under the force of the compression coil spring. When the support lever is moved away from the fixed clamp member, the movable clamp member is disengaged from the vertical edge of the door.
The work carrier of the foregoing construction is not fully satisfactory in that when subjected to a relatively large horizontal force occurring at the starting and stopping of the work carrier, the door is likely to be displaced from a desired clamped position against the force of the compression coil spring, resulting in inaccurate assembly of the vehicle door. An attempt to increase the spring constant of the compression coil spring may damage the vehicle door.
The work carrier disclosed in JP 1134877 A includes, as shown in FIGS. 7A to 7C hereof, a rectangular frame-like hanger 101 on which a fixed clamp unit 102, retainer members 103, 104 and a movable clamp unit 105 are arranged so that a vehicle door 106 can be clamped at opposite vertical edges thereof by and between the fixed clamp unit 102 and the movable clamp unit 105 with a lower edge thereof supported on the retainer members 103, 104. The movable clamp unit 105 includes a horizontally elongated support plate 108 mounted to a vertical frame of the hanger 101, and a slider 111 (FIG. 7B) slidably mounted on a guide member 109 attached to the support plate 108. An operation lever 112 is pivotally connected to the slider 111 and includes a screw member 113 connected to a lower end thereof for meshing engagement with a screw member 114 (FIG. 7B) provided on the support plate 108. The operation lever 112 is normally urged by a torsion coil spring 112a (FIG. 7B) in a direction to move the screw member 113 into meshing engagement with the screw member 114 on the support plate 108. The slider 111 is connected to a movable clamp member 115.
With this arrangement, when a vehicle door 106 is to be carried on the work carrier 100, the operation lever 112 is turned counterclockwise in FIG. 7B to thereby disengage the screw member 113 from the screw member 114 on the support plate 108. The slider 111 can now be moved along the guide member 109. While keeping this condition, the operation lever 112 is displaced in a direction parallel to an axis of pivotal movement thereof so that the movable clamp member 115 connected to the slider 111 is moved away from the fixed clamp unit 102. After the vehicle door 106 is placed on the retainer members 103, 104 with its one vertical edge held by the fixed clamp unit 102, the operation lever 112 is displaced in the opposite direction to move the slider 111 along the guide member 109 until the movable clamp member 115 comes into engagement with an opposite vertical edge of the vehicle door 106. Then a grip on the operation lever 112 is released whereupon the operation lever 112 is allowed to return to its original position (indicated by the solid lines shown in FIG. 7B) under the force of the torsion coil spring 112a, causing the screw member 113 to move into meshing engagement with the screw member 114 on the support plate 118. Thus, the movable clamp member 115 is locked in position against movement relative to the guide member 109, and the vehicle door 106 is clamped between the fixed and movable clamp units 102 and 105. Reference numeral 107 shown in FIG. 7A denotes another vehicle door of enlarged size that can be carried on the work carrier 100.
However, a problem may occur when the screw member 113 on the operation lever 112 comes into meshing engagement with the screw member 114 on the support plate 108. Especially when the screw member 113 approaches the screw member 114 with its tooth tips 116 (only one shown by phantom lines D shown in FIG. 7C) aligned with tooth tips 117 of the screw member 114, the tooth tips 117 of the screw member 113 move to slide either along left-side tooth flanks of the screw member 114 as indicated by the arrow E, or along right-side tooth flanks of the screw member 114 as indicated by the arrow F, as the screw member 113 is forced toward the screw member 114 by the force of the torsion coil spring 112a. As a consequence, the screw member 113 is axially displaced relative to the screw member 114 either in a direction indicated by the arrow G or in a direction indicated by the arrow H.
In the former case, since the movement of the screw member 113 in the direction of the arrow G is limited by the vehicle door 106 clamped between the fixed and movable clamp units 102 and 105, the tooth tips 116 of the screw member 113 stop moving along the left tooth flanks of the screw member 114 before they reach tooth bottoms 118 of the screw member 114. Due to insufficient meshing engagement between the screw members 113 and 114, the movable clamp unit 105 cannot firmly hold the vehicle door edge, allowing the vehicle door 106 to become displaced or wobbled when subjected to a relatively large horizontal force during conveyance by the work carrier 100. Alternatively, when the screw member 113 is displaced in the direction of the arrow H, there is created between the movable clamp member 115 and the mating edge of the vehicle door 106 a space corresponding in amount to one-half pitch of the screw member 113. The thus created space allows horizontal movement or wobbling of the vehicle door occurring during conveyance by the work carrier, which will hinder smooth and accurate assembling of a window glass and other parts relative to the vehicle door.
It is accordingly an object of the present invention to provide a work carrier, which is capable of carrying a plate-like work, such as vehicle door, with the work held in a firmly clamped condition without causing objectionable displacement or wobbling during conveyance thereof.
Another object of the present invention is to provide a work clamping method using the work carrier.